Electron optical system



Dec. 14, 1965 K. SCHLESINGER 3,223,871

ELECTRON DPTICAL SYSTEM Filed Aug. 22, 1961 3 Sheets-Sheet 1 FIG.I.

so IIIIIIMII- MODULATING susmu.

INVENTORI KURT SCHLESINGER SATTORNE Dec. 14, 1965 K. SCHLESINGER3,223,871

ELECTRON OPTICAL SYSTEM Filed Aug. 22, 1961 3 Sheets-Sheet 2 I3Ill/111/1 1 1 1,

INVENTORI KURT SCHL SINGER fig;

HIS

TORNEY.-

Dec. 14, 1965 K. SCHLESINGER 3,223,871

ELECTRON OPTICAL SYSTEM Filed Aug. 22, 1961 3 Sheets-Sheet 3 INVENTOR:URT SCHLESINGER United States Patent C 3,223,871 ELECTRON OPTHIAL SYSTEMKurt Schlesinger, Fayetteville, N .Y., assignor to General ElectricCompany, a corporation of New York Filed Aug. 22, 1961, Ser. No. 133,22113 Claims. (Cl. 31383) The present invention relates to ultra highresolution electron optical systems, and particularly to improvedcathode ray tubes embodying such electron optical systerns.

One object of the invention is to provide an electron optical system forproducing an electron beam having a tar-get spot size which is extremelysmall, e.g., a few microns in diameter, at target beam current levelsproviding desirable energy transfer to the target, e.g., severalmicroamperes.

Another object is to provide such an electron optical system wherein theelectron beam can be readily deflected through a substantialtarget-scanning angle, e.g., of the order of 40 degrees, and where-inthe overall physical length of the electron optical system isconveniently small.

Another object is to provide an electron optical system of the characterdescribed wherein defiection-defocusing of the electron beam isminimized.

Another object is to provide an electron optical system of the characterdescribed including convenient means for precisely controlling thecross-sectional shape of the target scanning electron beam.

Another object is to provide an electron optical system of the characterdescribed which does not require critical mechanical alignment of thestructural elements thereof.

Another object is to provide an improved cathode ray tube embodying anelectron optical system of the foregoing character.

These and other objects of the invention will be apparent from thefollowing description and the accompanying drawings wherein:

FIGURE 1 is an axial sectional view of a cathode ray tube constructedaccording to my invention;

FIGURE 2 is an enlarged fragmentary view of a portion of the structureof FIGURE 1;

FIGURE 3 is an enlarged transverse sectional view of the structure ofFIGURE 1 taken on the line 33 thereof;

FIGURE 4 is an enlarged transverse sectional view of the structure ofFIGURE 1, taken on the line 4-4 thereof;

FIGURE 5 is a view similar to FIGURE 4 showing an alternative form ofone feature of the invention;

FIGURE 6 is a schematic diagram of circuitry associated with theapparatus of FIGURE 5.

An electronic optical system constructed in accordance with my inventionis shown in FIGURE 1 embodied in a cathode ray tube which includes anenvelope having an elongated neck 2, and an enlarged funnel portion 4closed by a face plate 6 on the interior of face plate 6 is a target inthe form of a luminescent screen 8. The screen 8 may consist preferablyof a continuous transparent film of luminescent material which may be ofthe vapor-phase deposited type such as taught by US. Patents 2,675,331,2,685,530, and 2,887,401, commonly assigned herewith.

Arranged in order coaxially Within the neck portion of the envelope fromthe base or rearward end of the neck toward the target screen 8 are aplurality of sections hereinafter to be described in greater detail.These sections include an electron beam generating section 10, aprefocusing lens section 20, an electrically neck-elongating section 30,and a main focus lens section 40.

Referring to FIG. 2 in the electron beam generating section 10 of thetube, electrons are emitted from an 3,223,871 Patented Dec. 14, 1965axially located cathode 11 which is preferably of the highcurrent-density type such as a dispenser cathode. The emitted electronspass through axially apertured collimator electrode 12, first anode 13,beam intensity modulating gate electrode 14, and a meniscus electrode15. As best shown in FIGURE 2, anode 13 has a forwardly projecting(towards screen 8) convex surface 16 which, together with theconfronting rearwardly directly or facing concavity 17 of the gateelectrode 14 which, as illustrated, is in receiving relationship to theprojecting surface 16, form a focusing electrostatic field, theequi-potential surfaces of which are hyperboloids of revolutionsymmetrical with the axis of the tube neck and asymptotic to readwardlyconcave conical surface 17 of approximately 109 apex angle whose apexfaces screen 8 or away from said emitter and lies on the tube neck axisin substantial coincidence with the central aperture 18 in the meniscuselectrode. The characteristics of such a focusing electrostatic fieldare described in more detail in my copending application Serial No.16,523, now U.S. Patent 2,995,676 commonly assigned herewith.

The field between the anode 13 and the gate electrode 14 forms in theaperture 18 an effective virtual cathode of demagnified size relative tothe actual cathode 11 and thus illuminates the aperture 18 with anelectron beam of density many times that of the emission density fromthe actual cathode 11, for resulting maximum brightness at the screen 8.In a successfuly operated tube constructed as herein described, thecathode 11 was operated at ground potential, collomator 12 at about +10volts, anode 13 and meniscus 15 at about 500 volts, the beam intensitymodulation was achieved by varying the potential of gate 14 between 5and +10 volts, and current densities of about 25 amperes per squarecentimeter at aperture 18 were achieved with emission density at thecathode of only about 2 amperes per square centimeter.

The electron beam emerges from the aperture 18 into the prefocusing lenssection 20, within which is formed an electrostatic field whoseequipotentials are hyperboloids of revolution coaxial with the tube neckaxis, and asymptotic to and within a coaxial forwardly concave conicalsurface 21 of approximately 109 apex angle having its apex facingemitter 11 and located in substantial coincidence with aperture 18. Theforward end of the prefocusing lens section is terminated by atransverse axially apertured conductive wall 22 having a coaxial opening23 and spaced from the conical surface 21 by a supporting insulatingcylinder 24.

The two surfaces 21 and 22 serve to form the hyperbolic electrostaticfield within the prefocusing lens section 20, and, if desired, theformation of such field may be augmented by further electrode means,such as resistive coatings on the cylinder 24 having local potentialscorresponding to the local space potential of the prefocusing lensfield.

In FIG. 1, adjacent transverse wall 22, and closed at its rearward endthereby, is the electrically neck-elongating section 30 which includesan accelerating cylindrical spiral electrode 31 arranged coaxially withthe neck axis. The spiral electrode 31 may conveniently consist, asshown, of a conductive spiral coating of uniform pitch on the interiorsurface of an insulating support cylinder 32. Desirably the spiralelectrode 31 has a high impedance of the order of 3050 megohms tominimize power consumption. Wall 22 and the rearward end of the spiralelectrode 31 are connected by a lead 35 to a suitable adjustablepotential source, shown schematically as potentiometer 50, which mayprovide to lead 35 a relatively low potential of the order of to 800volts. A conductivecoating 33 on the exterior of the insulating cylinder32 serves as an electrostatic shield for the spiral electrode and alsoconveniently provides part of a conductive path from the forward end ofelectrode 31 to a conductor 53 of substantially higher potential, whichmay be for example of the order of 7,000 volts, so as to provide asubstantial accelerating field within spiral electrode 31. Annularconductive caps 27, 36 are provided at each end of the cylinder 32 tofacilitate mechanical support and provide convenient electricalconnections to the ends of spiral electrode 31.

The forward end of the cylinder 32 is closed by a transverse conductivewall 37 having a central aperture 38. Forward of the aperture 38 andpartially supported by wall 37 is the main focusing lens section at) ofthe tube, which may be of any suitable type including a unipotentiallens, but is here shown as a bipotential lens. Section 40 includes asone element of the bipotential lens a coaxial conductive cylinder 41 ofdiminished diameter relative to electrode 31 and having an enlargedmouth 42 at its forward end. Cylinder 41 preferably has the samepotential as the forward end of spiral electrode 31. The aperture 38serves as a limiting aperture minimizing aberration through the lenssection 40, and another transverse wall 43 intermediate the ends ofcylinder 41 has an axial aperture 44 and serves as a shield to cut downemission of stray electrons from the lens section 40. The forward end ofthe lens section 40 is spaced by an annular insulator 45 from asupporting conductive sleeve 46 which in turn is connected by conductivesupport fingers 47 to the neck wall. Sleeve 46 forms the second elementof the bipotential lens and is electrically connected by fingers 47 andan internal conductive coating 48 at the front of the neck and on theinner surface of the envelope funnel 4 to the high voltage terminal 51of the tube, which may have a potential several times that of cylinder41, for example 20 kv. A suitable deflection yoke 49 is provided forscanning the electron beam on the screen 8.

In the operation of the electron optical system, an electron beam ofhigh current density, for example of the order of 25 amperes per squarecentimeter, substantially demagnified in cross-section by the focusingfield between anode 13 and gate 14, and modulated in intensity by acontrol signal applied to gate 14, is supplied to aperture 18, and formsthere a virtual cathode. The prefocusing lens section 20 operates toprovide a virtual image of the virtual cathode at aperture 18, whichvirtual image serves as the object for the main focusing lens 40. It hasbeen found that the accelerating section 30 has the electrical effect ofelongating the distance from the principal plane of the main focusinglens 40 to its effective object plane. Thus, since the magnification ofthe main focusing lens, as is well known to those skilled in the art, isproportional to the image distance divided by the effective objectdistance, for a fixed image distance between the screen 8 and the mainfocusing lens 40 the image size or electron beam spot size on the screenis correspondingly decreased by the neck-elongating or objectdistance-elongating action of spiral accelerating electrode 31.

Dynamic focusing as well as control of resolution is providedconveniently by varying the potential of the wall 22 relative to thepotential of surface 21 of the meniscus electrode 15, which in turnmodifies the action of the prefocusing lens section 20. For example, ithas been found that as the potential of wall 22 is increased above thatof surface 21, the overall effect of the prefocus lens becomes positiveor converging, and the virtual image of the aperture 18 produced by theprefocus lens section moves rearwardly and increases in size. Since thisvirtual image serves as the effective object for the main focusing lens40, the overall effect at the screen is one of moderate spot sizeenlargement with a considerable increase in beam current, such that thecurrent density observed at the screen remains substantially constant.When the potential of wall 22 equals that of surface 21 of meniscuselectrode 15, the space between meniscus electrode 15 and wall 22becomes substantially field-free, and the location of the effectiveobject for the main focusing lens is at the aperture 18. When thepotential of wall 22 is decreased below that of the meniscus electrode,however, it has oeen found that the overall effect of the prefocusinglens section 20 is diverging or negative, and the virtual image ofaperture 18 which the prefocus lens forms moves forward of the aperture18 and is diminished in size. The result of this at screen 8 is that theelectron beam spot size gets smaller and beam current decreasessomewhat. Thus convenient control of resolution, as well as dynamicfocusing, is obtained merely by adjusting the potential of Wall 22.

I have found that, with a potential at terminal 51 of 20 kv., withcylinder 41 and the forward end of spiral electrode 31 at 7 kv., andwith meniscus electrode 15 at 500 volts, in a tube of the typedescribed, excellently small spot sizes of about .00033 inch (8 microns)at screen 8 with 1.5 microampere beam current can be obtained. This spotsize is obtained when the field in the prefocusing lens 20 isdecelerating, and the rearward end of spiral electrode 31 and wall 22have a potential of about 0.6 that of electrode 15, i.e., 300 volts.

Optionally, the neck-elongating section 30 may be arranged to act as amoderate converging lens for the electron beam, as well as anaccelerator, for example by changing the spiral 31 from a uniform pitchto one having a pitch progressively increasing toward wall 37. Howeverin such a case, care must be taken that the converging lens action ofthe section 30 is not made so strong as to produce a second crossover ofthe electron beam before it reaches the screen 8, since such a resultwould cause undesired enlargment of the spot size at the screw.

Since proper centering of the electron beam at aperture 18 and at thelimiting aperture 38 at the forward end of the spiral electrode 31 isimportant to preserve efficient transmission of beam current through theelectron optical system, to alleviate problems of exact coaxialalignment of the various parts of the system as well as to correct forthe effect of the earths magnetic field, adjustable centering coils areprovided external to the tube neck in the vicinity of the electron beamgenerating section 10, to provide centering at aperture 18. A similarset of coils 67 is provided in the vicinity of the spiral electrode 31to provide centering at aperture 38. To avoid repetition, only coils 60will be described in detail, it being understood that coils 67 aresimilar in all respects except for increased length, as is apparent fromFIG- URE 1.

As shown in the sectional view of FIGURE 3 the centering coil assembly60 outside section 10 consists of an insulating cylindrical support 61on which are mounted two pairs of electromagnetic coils, the windings ofthe coils being designated AA, BB, CC, and DD far clarity. Coils AA andBB are wound in series and supplied from a remote source ofreversible-polarity, adjustable amplitude direct current, which may befor example a battery 62 and a potentiometer 63, and are for centeringthe beam in the horizontal direction. Coils CC and DD are likewise woundin series and supplied from a remote source of reversible-polarityadjustable amplitude direct current, which as shown may be battery 62and a potentiometer 64, and are for centering the beam in the verticaldirection.

For the purpose of permitting precise Vernier control of thecross-sectional shape of the electron beam, in order to enhance spotroundness or if desired to produce spot ellipticity at the screen, anassembly of beam shaping coils is provided external to the tube neck inthe vicinity of the main focus lens section 40. This beam shaping coilassembly '70 is shown in detail in the cross-sectional view of FIGURE 4.It consists of an insulating support cylinder 71 which is rotatablyadjustable about the neck axis and on which are mounted four coils EE,FF, GG, and HH each arranged to subtend a angle in a plane transverse tothe neck axis. The coils are wound in series in a sense such as todevelop two mutually perpendicular pairs of magnetic poles wherinadjacent poles are of opposite polarity and diametrically opposed poleshave the same polarity, as shown in FIGURE 4. The coils are connected toa remote source of reversible-polarity adjustable-amplitude directcurrent, which may be for example a potentiometer 64 and battery 62 asshown in FIG- URE 3.

Referring to FIGURE 4, the N and S negative poles there shown, whichresult from the current in coils EE, FF, GG, and HH, indicate how themutually orthogonal forces of an electron beam passing in an axialdirection through the field generated by coils EE, FF, GG and HH tend tocorrect ellipticity of the beam. A beam having an undesired degree ofallipticity, for example with a major axis 95, can be convenientlyrendered circular in cross-section merely by properly rotating the coilassembly 70 so that the beam compressing transverse magnetic fieldsthereof, as shown by poles N and S and vectors 72, coincide with thecompress the major axis of the ellipse, and the beam expandingtransverse magnetic forces thereof, as shown by vectors 73, coincidewith and expand the minor axis of the ellipse, and adjusting the currentthrough the coils of assembly 70 for the desired degree of roundness ofthe beam. Conversely, if for any reason a desired degree of ellipticityis required, such may be obtained even from a beam of perfectly circularcross-section by suitably angularly adjusting the coil assembly andproperly adjusting the amount of current through the coils of assembly70.

It will thus be appreciated from FIGURE 4 that the coil assembly 70provides magnetic fields through the tube neck at the main focus lenssection 40 which cannot provide any deflection of the beam in a mannersuch as would affect its centering but which do distort or change asdesired the cross-sectional shape of the electron beam.

Alternative means for Vernier control of the crosssectional shape of theelectron beam are shown in FIG- URES-5 and 6. The apparatus of FIGURES 5and 6 has the advantage that it effects control of beam shape entirelyelectrically, eliminating any need for mechanical rotation of a coilassembly around the tube neck. This alternative beam shaping meansincludes an assembly of coils arranged external to the tube neck in thevicinity of the main focus lens section 40, and shown in detail inFIGURE 5. The coils of FIGURE 5 are arranged and energized to providemagnetic fields which produce pairs of force vectors which act along themajor and minor ellipse axes of the electron beam, similar to the actionof vectors 72 and 73 of FIGURE 4, except that with the apparatus ofFIGURES 5 and 6 the force vectors can be electrically rotated through360 to any desired angle of orientation, and the amplitude of each pairof vectors can be electrically varied from a maximum in one sensethrough zero to a maximum in the opposite sense. Thus the apparatus ofFIGURES 5 and 6 can correct for, or introduce, any degree of ellipticityof the beam at any angular orientation of the ellipse axes.

Turning to FIGURE 5, eight identical coils are provided in two sets offour-coils each, designated A1, A2, A3, A4, and B1, B2, B3, B4. Thecoils of the A set subtend adjacent angles of 90 at the neck axis, andthe coils of the B set likewise subtend adjacent angles of 90 but aredisplaced 45 relative to the A coils. Each of the two current paths ofeach coil which extends parallel to the neck axis is centered in asector subtending an angle of 22 /2" at the neck axis. Designating thecurrent flow directions by plus and minus signs, the coil arrangementwill be evident from FIGURE 5. All four A coils are wounds in series andare supplied by direct current of controllable amplitude and reversiblepolarity, and all four B coils are wound in series and supplied bydirect current of controlla'ble amplitude and reversible polarity,separately from the A coils. The diametrically spaced coils of each set,e.g., coils A2 and A4, provide magnetic poles of one polarity, say theNorth poles designated NA in FIGURE 5,

while the remaining coils A1 and A3 of the same set provide poles ofopposite polarity, shown as SA in FIGURE 5. Such poles NA and SAcorrespond in function and eflect to the N and S poles of FIGURE 4.Likewise the B coils provide poles SB and NB of FIG- URE 5. Thus theeight coils are capable of providing eight magnetic poles spaced 45about the axis of the electron beam. The polarity of any diametricallyspaced pair of poles is always identical, and the polarity of any spacedpoles is always different, so that regardless of the eifect onellipticity there is no net effect on the centering of the electronbeam. Moreover, by the circuit of FIGURE 6 hereafter to be described,the strength and polarity of each such pole may be adjusted from maximumin one sense through zero to maximum in the opposite sense, and thus thesummed values of the mutually perpendicular pairs of force vectorsexerted on the electron beam by such poles may be made to rotate to anyangle and have any desired amplitude.

The circuit of FIGURE 6 includes a pair of field intensity controlpotentiometers 105, 113, which provide for control of the amplitude ofthe current fed to the A and B coils, respectively, and a pair of fieldorientation control potentiometers 103, 111. As will be explained indetail hereinafter, the potentiometers 103 and 111 serve to impose asine-cosine relationship on the variations in current amplitude andpolarity supplied the respective A and B coil sets, and this enablescontrol of the angular orientation of the summed force vectors producedby the magnetic poles of FIGURE 5, throughout the entire 360.

As shown in FIGURE 6, one side of the A coil series is grounded, and theother side is connected through a current limiting resistor 101 andfield orientation control potentiometer 103 to the field intensitycontrol potentiometer 105 which with battery 107 provides a source ofreversible polarity adjustable amplitude direct current. Likewise oneside of the B coil series is grounded and the other side is connectedthrough resistor 109 and orientation control potentiometer 111 to asource of reversible polarity adjustable amplitude direct current atintensity control potentiometer 113. The movable contacts 140, 141 ofpotentiometers 105, 113 are ganged, and their stationary windings areconnected to battery 107 with opposite polarity.

Potentiometer 103 has four equal segments separating five terminals 115,123, 124, 125, 126, and potentiometer 111 likewise has four equalsegments separating five terminals 116, 127, 128, 129, 130. The movablecontacts 135, 136 of potentiometers 103 and 111 are ganged.

The terminals 115', 124 and 126- of potentiometer 103 and the terminals128 and 130 of potentiometer 111 are grounded, while terminals 123 and129 are joined, and 127 are joined, and 116 and 126 are joined.

Intensity control potentiometer 113 determines the maximum current ofone polarity available to the B coils from the terminal 129 ofpotentiometer 111 and available to the A coils from terminal 123 ofpotentiometer 103, while potentiometer 105 controls the maximum currentof the opposite polarity available to the A coils from terminal 125 ofpotentiometer 103 and available to the B coils from terminal 127 ofpotentiometer 111.

As the movable contacts 135, 136 of potentiometers 103 and 111 are movedthe currents to the A and B coil sets available from contacts 135, 136vary from zero to a maximum in one sense, through zero again and to amaxim-um of the opposite sense with a 90 phase dilference and hence havea continuous relative approximately sinecosine relationship. Thesesine-cosine related currents produce vector summed magnetic fields whichcan be rotated to any angular orientation in FIGURE 5, and thus thesetting of the movable contacts of potentiometers 103 and 111 enablesthe summed force vectors affecting the ellipticity of the electron beamto be rotatively oriented at any angle throughout the 360 range.Moreover the settings of potentiometers 103, 113 control the maximumvalues of the currents available from contacts 135, 136, and hencecontrol the amplitude of the force vectors affecting the ellipticity ofthe electron beam. Thus merely by adjusting the pairs of moveablecontacts 140, 141, and 135, 136, any degree of ellipticity may beintroduced into or removed from the electron beam, without affecting itscentering.

Thus there has been show-n and described a high resolution electronoptical system for a cathode ray tube or the like which permits thegeneration of extremely fine electron beam spot sizes of as little as afew microns diameter and within an overall length which is convenientlysmall. The mechanical structure of a cathode ray tube embodying suchelectron optical system is relatively simple and rugged, and associatedmeans surrounding the axis of the system is provided for obtainingprecise centerlng of the electron beam and thereby relaxing thetolerances of alignment of the parts. Additionally precise verniercontrol of the beam cross-sectional shape is provided so thatsubstantial scanning angles of the order of 40 can be obtained withminimum defocusing.

A cathode ray tube constructed as above described has been found toprovide very large screen current density of the order up to 2.0 amperesper square centimeter, with screen power loadings of up to approximately50 kw. per square centimeter at 20 kw. screen potential and with aresolution of up to the equivalent of 10,000 raster lines on a tubeface. Such resolution is the equivalent of up to 400 complete TVpictures arranged in a square or up to 100 million dots or 25 millionhits of four increments per bit.

It will be appreciated by those skilled in the art that the inventionmay be carried out in various ways and may take various forms andembodiments other than those illustrative embodiments heretoforedescribed. Accordingly it is to be understood that the scope of theinvention is not limited by the details of the foregoing description,but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electron optical system comprising, arranged in coaxial relation,an electron beam source, a target spaced from said source, a mainelectron beam focusing lens disposed between said source and saidtarget, an electron beam prefocusing electron lens disposed between saidelectron beam source and said target for providing an electron opticalimage of said electron beam source for imaging on said target by saidmain focusing lens, and accelerating spiral electrode means providing anaxial electron accelerating field between said prefocusing lens and saidmain lens, and means for varying the strength of said accelerating fieldto vary the effective object distance of said main lens to reduce spotsize on said target.

2. An electron optical system comprising, arranged in coaxial relation,an electron beam source, a target spaced from said source, a mainelectron beam focusing lens disposed between said source and saidtarget, cylindrical spiral accelerating electrode means providing anaxial electron accelerating field between said main focusing lens andsaid electron beam source, a prefocusing electron lens between saidelectron beam source and said accelerating electrode means for providingan electron optical image of said electron beam source for imaging onsaid target by said main focusing lens, said accelerating electrodemeans having a planar transverse apertured wall portion at one endthereof next adjacent said prefocusing electron lens, and electron beamdeflecting means between said main focusing lens and said target forscanning said beam across said target.

3. An electron optical system comprising, arranged in coaxial relationalong a reference axis, an electron beam source including an emitter,electrode means forming an electrostatic field having equipotentialsurfaces which are hyperboloids asymptotic to a conical surface ofapproximately 109 apex angle coaxial with said reference axis and withsaid apex facing away from said emitter, a target spaced from saidsource, a main electron beam focusing lens disposed between said sourceand said target, spiral accelerating electrode means providing an axialelectron accelerating field between said main focusing lens and saidelectron beam source, and a prefocusing electron lens between saidsource and said accelerating electrode means for providing an electronoptical virtual image of said electron beam source for imaging on saidtarget by said main focusing lens.

4. An electron optical system comprising, arranged in coaxial relation,an electron beam source, a target spaced from said source, a mainelectron beam focusing lens disposed between said source and saidtarget, axially extending accelerating electrode means providing anaxially extending uniform axial electron accelerating field between saidmain focusing lens and said electron beam source, and a prefocusingelectron lens between said source and said accelerating electrode meansfor providing an electron optical image of said electron beam source forimaging on said target by said main focusing lens, said prefocusingelectron lens forming an electrostatic field having equipotentialsurfaces which are hyperboloids asymptotic to a forwardly concaveconical surface of approximately 109 apex angle coaxial with said axiswith said apex facing said electron beam source.

5. In an electron optical system including a reference electron beamaxis, coaxial prefocus lens means forming an electrostatic field havingequipotential surfaces which are hyperboloids asymptotic to a forwardlyconcave conical surface of approximately 109 apex angle coaxial withsaid axis, coaxial cylindrical spiral electrode means extending axiallyfrom said prefocus lens from the larger opening of said conical surfaceand forming a uniform axial electron accelerating field, and an axiallyapertured transverse electrode electrically connected to said spiralelectrode separating said prefocus lens and said spiral electrode, andmeans for adjusting the potential of said transverse electrode.

6. In an electron optical system including an electron beam source and atarget spaced from said source along a reference axis, said electronbeam source including an anode electrode and a gate electrode, saidanode electrode having a forwardly projecting convex forward surfacecoaxial with said reference axis and said gate electrode having arearwardly directed concavity coaxial with said axis and centrallyapertured for passage of said electron beam, said gate electrode beingadapted to have applied thereto modulation signals for varying the flowof electrons in said beam, variable refractivity prefocus lens meanscoaxially disposed between said target and said beam source, andincluding an electrode having a forwardly concave conical surfacecoaxial with said reference axis and with the apex thereof facing saidelectron beam source, means forming an axial opening in said conicalsurface through which the electron beam from said source is adapted to.pass, electrode means forming a coaxial main focus lens intermediatesaid .prefocus lens and said target, and axially extending spiralelectrode means forming an axially extending uniform axial electronaccelerating field between said prefocus lens and said main focus lens,said axially extending electrode means having a transverse wallelectrode portion adjacent said prefocusing lens means.

7. In an electron optical system including an electron beam source and atarget spaced from said source along a reference axis, said electronbeam source including an anode electrode and a gate electrode, saidanode electrode having a forwardly projecting convex surface coaxialwith said reference axis and said gate electrode having a rearwardlydirected concavity and coaxial with said axis and centrally aperturedfor passage of said electron beam, said gate electrode being adapted tohave applied thereto modulations signals for varying the flow of elec- 9trons in said beam, variable refractivity prefocus lens means coaxiallydisposed between said target and said beam source, electrode meansforming a coaxial main focus lens intermediate said prefocus lens andsaid target, and axially extending electrode means defining an axiallyextending electron accelerating field between said prefocusing lens insaid main lens for electrically varying the effective object distance ofsaid main lens, said means including an electrode effective forextending the electronic length of said system between said prefocusinglens and said main lens.

8. In an electron optical system including an electron beam source and atarget spaced from said source along a reference axis, prefocus lensmeans forming between said target and said beam source an electrostaticfield having equipotential surfaces which are hyperboloids asymptotic toa forwardly concave conical surface of approximately 109 apex anglecoaxial with said reference axis with the apex of said conical surfacefacing said electron beam source, means for directing said electron beamaxially into said prefocus lens field, electrode means forming a coaxialmain focus lens intermediate said prefocus lens and said target, andaxially extending electrode means forming an axially extending electronaccelerating field between said prefocus lens and said main focus lens,said axially extending electrode means being effective to electronicallyextend the distance in said optical system between said prefocusing andsaid main lens, the said axially extending electrode having a transverseapertured wall electrode portion on at least one end thereof.

9. In an electron optical system including an electron beam source and atarget spaced from said source along a reference axis, said electronbeam source including an anode electrode and a gate electrode, saidanode electrode having a forwardly projecting convex surface coaxialwith said reference axis, said gate electrode having a rearwardlydirected concavity coaxial with said axis, said concavity beingcentrally apertured for passage of said electron beam, said gateelectrode being adapted to have applied thereto modulation signals forvarying the flow of electrons in said beam, prefocus lens means formingbetween said target and said beam source an electrostatic field havingequipotential surfaces which are hyperboloids asymptotic to a forwardlyconcave conical surface of approximately 109 apex angle coaxial withsaid reference axis with the apex of said conical surface facing saidelectron beam source, means for directing said electron beam axiallyinto said prefocus lens field, electrode means forming a coaxial mainfocus lens intermediate said prefocus lens and said target, and axiallyextending cylindrical electrode means forming a uniform axiallyextending electron accelerating field between said prefocus lens andsaid main focus lens, said axially extending cylindrical electrode meanshaving a coaxially apertured transverse Wall portion at each endthereof.

10. In an electron optical system including an electron beam source anda target spaced from said source along a reference axis, said electronbeam source including an anode electrode and a gate electrode, saidanode electrode having a forwardly projecting convex surface coaxialwith said reference axis, said gate electrode having a rearwardlydirected concavity coaxial with said axis, said concavity beingcentrally apertured for passage of said electron beam, said gateelectrode being adapted to have applied thereto modulation signals forvarying the flow of electrons in said beam, prefocus lens meanscoaxi-ally disposed between said target and said beam source andincluding an electrode having a forwardly concave conical surface ofapproximately 109 apex angle coaxial with said axis and with the apexthereof facing said electron beam source, an axially aperturedtransverse electrode axially spaced forward of said conical surface,means forming an axial virtual cathode opening in said conical surface,means for directing said electron beam axially through said virtualcathode opening, electrode means forming a coaxial main focus lensintermediate said prefocus lens and said target, and a coaxialcylindrical spiral electrode extending between said prefocus lens andsaid main focus lens for forming a uniform axial accelerating field.

11. An electron optical system comprising, arranged in coaxial relation,an electron beam source, a target spaced from said source, a mainelectron beam focusing lens disposed between said source and saidtarget, prefocusing electron lens means between said source and saidm-ain lens for providing an electron optical virtual image of saidelectron beam source for imaging on said target by said main lens,electrical means for controlling the effective object distance of saidmain lens, electron beam ellipticity control means includingelectromagnetic coil means for forming a plurality of transversemagnetic fields in the path of said beam and having diametrically spacedpairs of magnetic poles quadrilaterally spaced in a plane transverse tosaid beam, the poles of each diametrically spaced pair being of the samepolarity and the adjacent poles of each quadrilaterally spaced set beingof opposite polarity, and means for varying the strength and polarity ofsaid poles.

12. Apparatus as defined in claim 11 wherein said pole strength andpolarity varying means includes a support for said coil means rotatableabout said reference axis, and a source of variable magnitude reversiblepolarity direct current for said coil means.

13. Apparatus as defined in claim 11 wherein said magnetic coil meansincludes eight coils arranged to provide said quadrilaterally spacedpoles in two sets of four each with the poles of one set beingrotatively displaced 45 relative to the poles of the other set, andwherein field intensity control means are provided to control themagnitude of the current maximum current available to each coil, andwherein field orientation control means is provided to impose asubstantially sinecosine relationship on the variation of pole strengthand polarity between the coils of one set relative to the coils of theother set.

References Cited by the Examiner UNITED STATES PATENTS 2,318,423 5/1943Samuel 31514 X 2,520,813 8/1950 Rubenberg 31515 2,630,544 3/1953 Tiley315--3.6 2,914,675 11/1959 Van Dorsten. 2,986,668 5/1961 Haflinger etal. 31383 X 2,995,676 8/ 1961 Schlesinger 31515 3,040,205 6/ 1962 Walker31383 X FOREIGN PATENTS 735,463 8/ 1955 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner.

JOHN W. HUCKERT, GEORGE N. WESTBY,

Examiners.

1. AN ELECTRON OPTICAL SYSTEM COMPRISING, ARRANGED IN COAXIAL RELATION,AN ELECTRON BEAM SOURCE, A TARGET SPACED FROM SAID SOURCE, A MAINELECTRON BEAM FOCUSING LENS DISPOSED BETWEEN SAID SOURCE AND SAIDTARGET, AN ELECTRON BEAM PREFOCUSING ELECTRON LENS DISPOSED BETWEEN SAIDELECTRON BEAM SOURCE AND SAID TARGER FOR PROVIDING AN ELECTRON OPTICALIMAGE OF SAID ELECTRON BEAM SOURCE FOR IMAGING ON SAID TARGET BY SAIDMAIN FOCUSING LENS, AND ACCELERATING SPIRAL ELECTRODE MEANS FORPROVIDING AN AXIAL ELECTRON ACCELERATING FIELD BETWEEN SAID PREFOCUSINGLENS AND SAID MAIN LENS, AND MEANS FOR VARYING THE STRENGTH